Magnets and magnetic fields have long been used for separating ferrous (iron-containing) metals from bulk waste or from a conveyed waste stream. An example of such a use may be found at automotive shredding facilities where scrap autos are disposed of, often at the rate of 300-400 autos per day.
When disposing of a scrap auto using a shredder, common practice is to remove the tires, battery and the fluids, e.g., anti-freeze and motor oil, and break up or "reduce" the remainder in a shredder, hammermill or the like. The auto being reduced into much smaller pieces of scrap will include both ferrous and non-ferrous metal constituents (such as steel and aluminum, respectively) as well as non-metal constituents such as plastics and fabric. After such reduction, the scrap is fed to a conveyor belt, the discharge end of which is equipped with a drum-like magnetic separator for "first-stage" separation. Ferrous metal "follows" the contour of the separator drum through about a 180.degree. path and falls from the conveyor belt when such belt leaves the separator drum. The remaining scrap is "projected" somewhat forward of the separator by the moving conveyor belt to a "downstream" second conveyor belt for subsequent removal of the non-ferrous metal from the non-metal constituents of the remaining waste stream.
Such non-ferrous metal constituents, e.g, aluminum, are removed by using what is known as an eddy current separator. Eddy current separators are discussed in U.S. Pat. Nos. 4,031,004 (Sommer, Jr. et al.); 4,069,145 (Sommer, Jr. et al.) and 4,668,381 (Julius).
A conventional eddy current separator has a number of relatively-small magnets arranged to form a drum-like assembly. Such assembly rotates at a speed of 1500 RPM to 3000 RPM and as the magnetic fields produced by such magnets "sweep across" the non-ferrous metal, a circulating electrical current or "eddy current" is induced in such metal.
Like all electrical currents, such eddy current produces a magnetic field having a polarity which is the same as that of the magnet which induced such current. Since like magnetic poles repel one another, the scrap metal piece is repelled and projected away from the conveyor belt along a fairly-predictable trajectory to a receptacle spaced somewhat away from the eddy current separator.
The remaining non-metal constituents (which consist largely of "fluff" from shredded upholstery) fall from the end of the conveyor belt to a receptacle adjacent to such separator where they are collected for removal. Eddy current separators are sometimes referred to as "flinger" separators since they literally fling non-ferrous metal pieces away from the conveyor belt.
And eddy current separators are not only used for separating non-ferrous metal from shredded autos. Such separators have great utility in separating non-ferrous metals, particularly aluminum beverage cans, from municipal waste streams. With the advent of "curbside" segregation of recyclable materials such as plastic beverage containers, tin-coated steel cans, glass and aluminum cans, eddy current separators are very useful to remove aluminum cans from such recyclable materials after the ferrous materials have been removed.
While known eddy current separators and ancillary equipment have been generally satisfactory, there are a number of problems that, until the invention, defied solution. One involves the eddy current magnet assembly which can be impacted by metal pieces piercing the conveyor belt with which the separator is operating. The magnet assembly is made of expensive and very-brittle (almost glass-like) rare earth magnets and represents a major portion, i.e., 50% or more, of the value of the separator.
To have a better understanding of this problem, it is helpful to appreciate two facts. One is that small vagrant ferrous pieces may remain in the waste stream even after first-stage "ferrous product" magnetic separation. The second is that because of the high speed at which the eddy current separator rotates, such vagrant ferrous pieces spin on the conveyor surface at high speed. These spinning pieces can (and often do) "drill" a hole in the conveyor belt, pierce the composite shell supporting the belt and fly into the separator magnets and fracture them. Resulting replacement cost is high and downtime is expensive.
One known magnet assembly is wrapped with resin-treated carbon filament threads. This arrangement is for retaining the magnets against centrifugal force and offers essentially no protection against projectile-like pieces which pierce the conveyor belt and the shell.
Another problem of known eddy current separators involves the durability of the above-mentioned cylindrical composite shell spaced from and surrounding the eddy current magnet assembly. Such shell contacts and supports the conveyor belt and rotates at relatively low speed. Conventional shells are made of fiberglass and do little to protect the magnet assembly spinning within. And if a ferrous particle lodges on the shell between the belt and such shell, the particle (which will spin for the reasons described above) can cut a groove in the shell. In an only-somewhat-more-extreme case, such a particle can sever the shell into two pieces.
Still another problem of known eddy current separators arises from the above-mentioned small ferrous pieces and dust-like "fines" remaining in the waste stream after first-stage separation. Such particles are not removed by the eddy current separator but, rather, tend to cling to the conveyor belt and fall from such belt at a point behind the separator. Therefore, a separate collection receptacle must be provided. Until the invention, there was no way to collect such particles together with the other non-metal constituents, e.g., auto upholstery "fluff," in the same receptacle.
Another problem arises from the conveyor belts used with conventional separators. Such belts have regularly-spaced, laterally-disposed cleats on the belt surface. Such cleats project well above the belt surface and because of their height, significant quantities of very small ferrous particles tend to collect on the cleats and, particularly, at the junction of the cleat edge and the belt. As noted above, such particles spin wildly when passing near the rotating separator magnet assembly and bore holes into and through the conveyor belt.
Yet another problem involves the magnetic structure of the magnet assembly itself. It is known that the magnetic effect between a magnet and, e.g., a non-ferrous piece of metal diminishes by the square of the distance between the magnet and the metal piece. And conventional magnets have flat pole faces. These facts suggest that the preferred way to construct a magnet assembly is to use a relatively large number of smaller magnets. Since the chord length of the pole face of each magnet is relatively short, such magnets can be positioned closer to the surrounding composite shell and, thus, closer to the conveyor belt. Such approach is used in known assemblies.
(This somewhat-difficult-to-visualize concept might be better understood by considering placing the ends of a straight stick chord-like against the inside surface of a barrel. The shorter the stick, the closer the stick center to such inside surface.)
The otherwise-diminuted strength of the magnetic field resulting from using small magnets is understood by designers of such magnet assemblies to be compensated by the larger number of magnets. But tests demonstrate that while field strength at the surface of the conveyor belt may be adequate, such field strength drops off rapidly at points progressively farther away from such belt.
Further, known magnet assemblies support the individual magnets on what is sometimes referred to as a back bar. A back bar is an elongate, tube-like structure concentric with the axis of rotation of the assembly. The bar has, for example, seven, eight or nine flat surfaces extending along the bar length. A bar with eight such surfaces would be octagonal in cross-section and magnets are mounted along the length of each such surface.
While the back bar is seemingly necessary, it occupies a good deal of volumetric space that could otherwise be occupied by magnet material. Until the invention, there was no way to eliminate the back bar and use the resulting space for magnets.
Eddy current separator features such as a well-protected magnet assembly, a well-protected outer shell, a structure to direct ferrous fines into a receptacle along with other waste, a conveyor belt with improved cleat arrangement, a high-mass magnet and a unique pole face for reducing air gap would be important advances in the art.